Heretofore, for membranes of electrolysis of sodium chloride, for membranes or catalyst layers for polymer electrolyte fuel cells, it has been common to employ a polymer which is obtainable by hydrolyzing a copolymer of a fluoromonomer represented by CF2═CF—(OCF2CFY)m—Op—(CF2)n—SO2F (wherein Y is a fluorine atom or a trifluoromethyl group, n is an integer of from 1 to 12, m is an integer of from 0 to 3, and p is 0 or 1, provided that m+p>0) with tetrafluoroethylene, or a polymer having sulfonic acid groups obtainable by further converting the above polymer to an acid form (hereinafter a polymer having sulfonic acid groups will be referred to as a sulfonic acid polymer).
Such a sulfonic acid polymer has a softening temperature in the vicinity of 80° C. Accordingly, the operation temperature of a fuel cell employing such a polymer is usually at most 80° C. However, in a case where hydrogen obtainable by reforming an organic compound composed of carbon atoms and hydrogen atoms such as methanol natural gas or gasoline, or hydrogen obtainable by reforming an organic compound composed of carbon atoms, hydrogen atoms and oxygen atoms, is used as a fuel gas for a fuel cell, if a carbon monoxide is contained even in a trace amount, the electrode catalyst will be poisoned, and the output of the fuel cell tends to be low. Accordingly, in order to prevent such a trouble, it is desired to increase the operation temperature. Further, also with a view to downsizing the cooling device for fuel cells, it is desired to increase the operation temperature. However, the above-mentioned conventional polymer has a low softening temperature and cannot satisfy such demands.
On the other hand, use of a sulfonic acid polymer having short side chains and having a high softening temperature as an electrolyte for a fuel cell has been proposed (Patent Document 1). However, such a sulfonic acid polymer is difficult to produce and is expensive. Further, it has been proposed to provide a polymer having a higher tensile modulus at 110° C. or above than that of Nafion film (tradename, manufactured by E.I. duPont) which is the above-described sulfonic acid polymer, using a material having at least two fluoropolymer chain segments differing in the composition, at least one of which has a sulfonic acid type functional group (Patent Document 2). However, the effect of increasing heat resistance is insufficient, since a vinyl ether type monomer is used as the segment having a sulfonic acid group.
Further, as a process for producing a graft copolymer disclosed in Patent Document 2, a method utilizing radioactive rays has been known. A method has been known which comprises irradiating a membrane-shape substrate of e.g. a polytetrafluoroethylene (PTFE), a tetrafluoroethylene/hexafluoropropylene copolymer (FEP) or a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) with radioactive rays, and bringing the substrate into contact with styrene to form graft side chains to introduce sulfonic acid groups. However, various improvements have been made since no sufficient chemical durability was achieved. For example, a method of preventing break of the main chain during irradiation with radioactive rays by employing an ethylene-tetrafluoroethylene copolymer (PTFE) as the membrane-shape substrate (Patent Document 3) or a method wherein PTFE is used as the membrane-shape substrate, and it is irradiated with radioactive rays while heated at 340° C. to crosslink PTFE, and the substrate is used in such a state (Non-Patent Document 1) may, for example, be mentioned.
However, in either case, the graft side chains comprise polystyrene sulfonic acid, and deterioration from the side chains is inevitable. Accordingly, a method of grafting side chains containing an oxyhydrofluorocarbon (—OCH2CF2CF2SO3H) to the crosslinked PTFE has been proposed (Patent Document 4), but it is difficult to secure sufficient durability because of C—H bonds.
Patent Document 1: Japanese Patent No. 2675548
Patent Document 2: JP-A-11-329062
Patent Document 3: JP-A-9-102322
Patent Document 4: JP-A-2002-348389
Non-Patent Document 1: Polymer Preprints, Japan Vol. 53, No. 2 (2004)